As previously described, the subject of consanguineous marriage became a source of major scientific and public interest in the UK and the USA from the mid-19th century onwards. Much of this interest centred on the claimed deleterious outcomes of consanguinity, although there were also scientists and clinicians who denied any adverse effects and instead argued that inbreeding offered major biological advantages. A large majority of studies have indicated that early mortality is increased in the progeny of consanguineous unions when compared with children born to unrelated parents. However, most of these studies failed to control for the potential effects of sociodemographic variables. Where such control was attempted, as in extensive post-World War II surveys conducted in Japan, the adverse biological effects of consanguinity were still present but were much less obvious, and similar findings have been reported in more recent studies in the Indian sub-continent.